Recently, in view of global environmental protection and cost effectiveness, self-ballasted fluorescent lamps with electrodes, which have about five times higher efficacy than that of incandescent lamps, have been widely used as substitutes for incandescent lamps in houses, hotels and other places. In addition to the already existing self-ballasted fluorescent lamps with electrodes, self-ballasted electrodeless fluorescent lamps have also been studied in recent years. A feature of electrodeless fluorescent lamps is that they have a longer life than fluorescent lamps with electrodes, owing to the absence of electrode. Electrodeless fluorescent lamps are thus expected to become widespread in the future.
Such a self-ballasted electrodeless fluorescent lamp is disclosed in Japanese Laid-Open Publication No. 10-92391, for example. The self-ballasted electrodeless fluorescent lamp disclosed in the publication is illustrated in FIG. 6.
The self-ballasted electrodeless fluorescent lamp 200 of FIG. 6 has as the entire device the shape of an incandescent lamp. More specifically, the lamp 200 is composed of a translucent discharge vessel 201, a coil 203 inserted in a cavity portion 201a of the discharge vessel 201, and a power supply circuit 204 for supplying alternating current to the coil 203. The coil 203 is made up of a rod-shaped ferrite core and a winding. The winding is connected to the power supply circuit 204. The power supply circuit 204 is formed and vertically placed on a circuit board on which a rectifier and a RF oscillator are provided in a vertical direction in the figure. The power supply circuit 204 is covered with a plastic case 205. Input power to the power supply circuit 204 is supplied via a base 207 provided on part of the case 205.
Mercury amalgam 206 and argon are enclosed as luminous substance in the discharge vessel 201, while a phosphor layer 202 is formed on the inner surface of the discharge vessel 201. The phosphor layer 202 changes ultraviolet light produced in the discharge vessel 201 into visible light.
However, to use electrodeless fluorescent lamps as substitutes for incandescent lamps, it is required to make the electrodeless fluorescent lamps closer to the incandescent lamps in terms of outer appearance and size. When a circuit board is placed vertically as in the above-mentioned disclosed electrodeless fluorescent lamp, it is difficult for the lamp to have an outer appearance and a size close to those of an incandescent lamp. Thus, in order to make the entire size almost equal to that of an incandescent lamp and then place the circuit board therein, the circuit board is preferably placed horizontally. In view of this, the present inventors have made an electrodeless fluorescent lamp in which a circuit board is placed horizontally and which is equal in size to an incandescent lamp.
The present inventors made various experiments using the lamp with the horizontally placed circuit board, and consequently found that when the lamp is operated, blackening is caused near the opening of the cavity portion of the discharge vessel and that the mercury reacts with the vessel wall and is consumed. Such blackening becomes particularly severe when a phosphor, a protective coating, or the like is not applied. The fact that blackening occurs in an inner tube around the winding of an induction coil has been conventionally known as disclosed in Japanese Laid-Open Patent Publication No. 11-102667. However, the fact that blackening occurs in the vicinity of the opening of the cavity portion was found by the present inventors for the first time. The mechanism behind the occurrence of blackening of the inner tube around the winding was that a high electric field, resulting from a potential difference between adjacent turns of the winding, causes ions or the like in plasma to be attracted to, and come into collision with, the tube wall. On the other hand, the blackening occurring near the opening of the cavity portion, which was found by the present inventors, is caused in the vicinity of a connection wire that extends from the coil, and cannot be explained by the mechanism disclosed in the above-mentioned publication, because there are no such adjacent turns. If such blackening occurs, the mercury is held in the blackened portion, which causes the problem that the quantity of mercury in the discharge gas decreases over the course of time, so that the quantity of emitted light is reduced. Nevertheless, since the mechanism behind such blackening is unknown, countermeasures cannot be taken easily.
In view of these circumstances, the present invention was made, and an object thereof is to provide a self-ballasted electrodeless fluorescent lamp in which no blackening occurs near the opening of a cavity portion of a discharge vessel.